1. Field of the Invention
The present invention relates to a tape tension controlling mechanism for use in a magnetic recording/reproducing apparatus, such as a videotape recorder (VTR).
2. Description of the Related Art
In recent years, there has been a growing demand for a decrease in size of magnetic recording/reproduction apparatuses, such as VTRs, camcoders, etc.
A tape tension controlling mechanism for use in a conventional magnetic recording/reproducing apparatus is described below.
As an example of a conventional technique for tape tension control, a mechanism disclosed in Japanese Patent Application No. 10-070169, which is incorporated herein by reference, is described with reference to FIG. 5, which is essentially equivalent to FIG. 15 of Japanese Patent Application No. 10-070169. In FIG. 5, reference numeral 1 denotes a cassette, and reference numeral 2 denotes a tape which is tensionally looped in the cassette 1. The tape 2 is wound around an S-reel (not shown) and a T-reel (not shown) contained in the cassette 1. For the clarity of illustration, only the outline of the cassette 1 and a portion of the tape 2 which is pulled out of the cassette 1 are represented by dashed lines in FIG. 5. The conventional magnetic recording/reproducing apparatus includes an S-reel bed 4 and a T-reel bed 5 which are engaged with, and rotate integrally with, the S-reel and the T-reel of the cassette 1, respectively. A tension arm 12 is rotatably supported by a tension arm shaft 13 mounted on a sub-chassis 3. The tension arm 12 has a tension post 14 at one end thereof, the tension post 14 projecting from a surface of the tension arm 12. Reference numeral 19 denotes a tension band, one end of which is axially supported by a shaft 20 of the tension arm 12. The other end of the tension band 19 is axially supported by a shaft 22 formed on a tension band restriction arm 21. A central portion of the tension band 19 is looped around a cylindrical portion 26 formed along the outer perimeter of the S-reel bed 4. Reference numeral 27 denotes an extension spring, one end of which is fixed to a spring fixing element of the sub-chassis 3. The other end of the extension spring is fixed to the tension arm 12. Accordingly, the tension arm 12 is urged in a counterclockwise direction by rotation about the tension arm shaft 13. The tension band restriction arm 21 is axially supported by a shaft 23 formed on the sub-chassis 3. The tension band restriction arm 21 is urged by a twisted coil spring 24 in a counterclockwise direction by rotation around the shaft 23. The tension band restriction arm 21 abuts a tension arm stoppage plate 116 so that the position of the tension band restriction arm 21 is controlled.
The magnetic tape 2 pulled out from the S-reel of the cassette 1 passes through a portion 2a and is looped around the tension post 14 with a predetermined contact area therebetween which corresponds to a predetermined angular distance of the magnetic tape 2 around the tension post 14. The magnetic tape 2 is looped around tape guide posts 49, 44, and 45, and a cylinder 38 having a rotatable magnetic head for the recording/reproducing of a signal to/from the magnetic tape 2 by running the magnetic tape 2 around a portion thereof.
In the above structure, when the magnetic tape 2 runs for recording or reproduction, frictional force is generated between the tension band 19 and the cylindrical portion 26 of the S-reel bed 4. At the same time, since a rotational moment is caused in the tension post 14 in the clockwise direction due to the tension of the magnetic tape 2, feedback is given to the frictional force so that the tape tension is controlled. Since the tension post 14 is a part of the running system of the magnetic tape 2, the tension post 14 must be accurately positioned. However, without the provision of an adjustment mechanism, it is difficult to accurately position the tension post 14 due to variations in the sizes of the various elements described above. Specifically, examples of the variations in sizes include a variation in the length of the tension band 19, a variation in the diameter of the cylindrical portion 26, a variation in the positions of the tension arm shaft 13 and the shaft 23, a variation in the position of the tension arm stoppage plate 116, etc. In this conventional example, such an adjustment mechanism is provided. Specifically, the tension arm stoppage plate 116 is fixed by a screw (not shown) to the sub-chassis 3 in such a way that the position thereof can be freely adjusted. With such an arrangement, the position of the tension band restriction arm 21 can be adjusted. As a result, by adjusting the length of the tension band 19 and by adjusting the position of the tension arm stoppage plate 116, the position of the tension post 14 can be freely adjusted.
However, the above structure involves the problems described below.
In order to adjust the position of the tension post 14, the following procedure steps are required to be performed. At the first step, the screw which tentatively fixes the tension arm stoppage plate 116 to the sub-chassis 3 is loosened. Then, the position of the tension arm stoppage plate 116 is adjusted thereby adjusting the position of the tension band restriction arm 21 so as to adjust the position of the tension arm 12, such that the tension post 14 is placed at a predetermined position. After the tension post 14 is appropriately placed at the predetermined position, at the last step, the screw is tightened so as to fix the tension arm stoppage plate 116 in place.
That is, the steps of loosening and tightening the screw are required for adjusting the position of the tension post 14. Further, the position of the tension post 14 is merely adjusted by trial-and-error while the screw is loosened, where during tightening the screw it is easy for the tension post 14 to move from the predetermined position, such that accurate positioning of the tension post 14 is not possible. Furthermore, it is necessary to form an external thread of the screw and an internal thread of a portion that receives the screw (these threads are generally formed by a tapping technique). Thus, in the conventional tape tension control mechanism, a large number of components are used; the adjustment performed during an assembly process is complicated and includes many steps; and it is difficult to reduce the size of the entire mechanism because an adjustment mechanism consumes a large space.